.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified 1993-03-29, David Metcalfe
.\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
.\" Modified 2003-10-25, Walter Harms
.\"
.TH ATEXIT 3  2008-12-05 "Linux" "Linux Programmer's Manual"
.SH NAME
atexit \- register a function to be called at normal process termination
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.BI "int atexit(void (*" function )(void));
.fi
.SH DESCRIPTION
The
.BR atexit ()
function registers the given \fIfunction\fP to be
called at normal process termination, either via
.BR exit (3)
or via return from the program's \fImain\fP().
Functions so registered are called in
the reverse order of their registration; no arguments are passed.

The same function may be registered multiple times:
it is called once for each registration.
.LP
POSIX.1-2001 requires that an implementation allow at least
.B ATEXIT_MAX
(32) such functions to be registered.
The actual limit supported by an implementation can be obtained using
.BR sysconf (3).
.LP
When a child process is created via
.BR fork (2),
it inherits copies of its parent's registrations.
Upon a successful call to one of the
.BR exec (3)
functions,
all registrations are removed.
.SH "RETURN VALUE"
The
.BR atexit ()
function returns the value 0 if successful; otherwise
it returns a nonzero value.
.SH "CONFORMING TO"
SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
.SH NOTES
Functions registered using
.BR atexit ()
(and
.BR on_exit (3))
are not called if a process terminates abnormally because
of the delivery of a signal.

If one of the functions registered functions calls
.BR _exit (2),
then any remaining functions are not invoked,
and the other process termination steps performed by
.BR exit (3)
are not performed.

POSIX.1-2001 says that the result of calling
.BR exit (3)
more than once (i.e., calling
.BR exit (3)
within a function registered using
.BR atexit ())
is undefined.
On some systems (but not Linux), this can result in an infinite recursion;
.\" This can happen on OpenBSD 4.2 for example, and is documented
.\" as occurring on FreeBSD as well.
.\" Glibc does "the Right Thing" -- invocation of the remaining
.\" exit handlers carries on as normal.
portable programs should not invoke
.BR exit (3)
inside a function registered using
.BR atexit ().

The
.BR atexit ()
and
.BR on_exit (3)
functions register functions on the same list:
at normal process termination,
the registered functions are invoked in reverse order
of their registration by these two functions.

POSIX.1-2001 says that the result is undefined if
.BR longjmp (3)
is used to terminate execution of one of the functions registered
.BR atexit ().
.\" In glibc, things seem to be handled okay
.SS "Linux Notes"
Since glibc 2.2.3,
.BR atexit ()
(and
.BR on_exit (3))
can be used within a shared library to establish functions
that are called when the shared library is unloaded.
.SH EXAMPLE
.nf
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void
bye(void)
{
    printf("That was all, folks\en");
}

int
main(void)
{
    long a;
    int i;

    a = sysconf(_SC_ATEXIT_MAX);
    printf("ATEXIT_MAX = %ld\en", a);

    i = atexit(bye);
    if (i != 0) {
        fprintf(stderr, "cannot set exit function\en");
        exit(EXIT_FAILURE);
    }

    exit(EXIT_SUCCESS);
}
.fi
.SH "SEE ALSO"
.BR _exit (2),
.BR exit (3),
.BR on_exit (3)
.SH COLOPHON
This page is part of release 3.44 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.